A Digital Subscriber Line (DSL) is a technology that effects digital communication to customers over the existing, twisted-pair telephone loop plant. The motivation behind the development of DSLs is the need to provide digital service to customers as part of the Integrated Services Digital Network (ISDN), which will become an international standard for digital communications. There are two kinds of ISDN access: Basic Access and Primary Access. The Basic Access signal is composed of up to 144 kbit/s of bidirectional customer data ("2B+D" channels), plus one bidirectional channel of 16 kbit/s to support provisioning and maintenance operations. This makes the total data rate of 160 kbit/s in each of two directions of transmission. Primary Access uses two cable pairs in a simplex fashion at 1.536M bit/s with a simplex of 8 kbit/s for a total transmission rate of 1.544M bit/s.
The current loop plant environment, including bridged taps and mixed metallic wire gauges, intended originally for voice frequency transmission, presents a complex environment for wideband transmission such as digital data services. In order to economically provide Basic Access, the DSL must be implemented without conditioning the loop plant (e.g., by removing bridged taps or rearranging pairs), notwithstanding the detrimental effects of bridged taps and gauge changes. Furthermore, no special engineering or operations can be associated with DSL installation.
As presently outlined in the Bellcore Technical Reference TR-NWT-000393, Issue 2, dated January 1991 and entitled "Generic Requirements for ISDN Basic Access Digital Subscriber Lines," which is incorporated herein by reference, the DSL is made up of a master digital transmitter/receiver ("transceiver") and a slave digital transceiver, both being connected by a metallic twisted-pair loop. The DSL uses the echo canceler-with-hybrid principle well-known in the art to provide full-duplex (i.e., simultaneous two-way) signal transmission over a two-wire non-loaded loop. The echo canceler technique is used to remove echoes of the transmitted signal that have mixed with the received signal; the echoes are reflections of the transmitted signal from discontinuities, such as bridged taps and gauge changes, or from line impedance mismatches and hybrid leakage. This permits a relatively weak received signal to be accurately detected and is the means for avoiding the use of a separate wire pair for each direction of transmission, thereby providing a cost-effective arrangement. A quaternary line code ("2B1Q") code is employed for signal propagation. This code converts blocks of two consecutive signal bits, representative of the signal generated by the customer, into a single four-level pulse for transmission. As a result, the loop baud is half of the information rate.
Primary Access currently uses the T1 line technology with a repeatered line operating at 1.544M bit/s. This technology requires special engineering of loops and removal of bridged taps and minimization of the occurrence of gauge changes. Exploratory work on the duplex system used in Basic Access suggested that it was possible to use a system similar to the Basic Access system operating at 800 k bit/s to provide a single Primary Access channel. Such an arrangement could also be used to provide service at 1.544M bit/s without special engineering, design or loop conditioning. The range of the system would be less than the Basic Access system because of the increased bandwidth with concomitant increased loss. However, it was determined that a greater range would be desirable if it could be achieved by improved signal processing rather than by loop conditioning. To achieve this range, it is especially important to improve the received signal-to-noise ratio.